Hydriding and dehydriding kinetics of Dy2Co7 hydride

Abstract

The rates of hydrogen desorption from the β- and γ-phases of Dy2Co7 hydride, as well as hydrogen absorption into the α- and β-phases were measured and compared. This represents the first time that such comparisons have been made of two-phase transitions within the same intermetallic hydride. Experiments were designed so that constant pressure thermodynamic driving forces were established and nearly isothermal conditions were maintained. Under these conditions, the rates of hydrogen evolution from the γ-phase were approximately 7 times faster than that from the β-phase. Hydrogen absorption by the β-phase was approximately 8 times faster than that by the α-phase. One possible explanation for the observed differences is that hydrogen atoms in the β- and γ-phases are in different types of lattice sites and this causes them to have different stability and mobility. This explanation would require that diffusion of hydrogen atoms within the crystal lattice have some role in the rate-controlling process. Enthalpies of formation, reaction orders, rate constants and activation energies were determined for each phase transition.